Aerosol liquid water: measurement, trends, and implications for atmosphere-biosphere interactions

Particle-phase liquid water is a ubiquitous and abundant component of atmospheric aerosols and influences hydrological cycling, fate and transport of trace species, visibility, human health, and regional climate. Aerosol water mass concentrations are functions of relative humidity (RH), temperature, and aerosol concentration and composition. Previous studies focused on inorganic species, and their relationships to particle hygroscopicity are included in atmospheric models. But there is a dearth of field studies and related ambient information pertaining to water modulation by organic compounds and vice versa. The extent to which water facilitates transfer of biogenically derived carbon from the gas to particle phase to form secondary organic aerosol (SOA), which contributes to the fine particle matter (PM2.5) burden, is poorly constrained. My dissertation aims to 1) better understand the chemical and thermodynamic controls on aerosol water in the Southeast U.S., an area known to contain high concentrations of SOA, through instrument development and field measurements, 2) quantify the temporal, spatial, and aloft trends in aerosol water through observational estimates, and 3) reconcile discrepancies between existing surface and satellite measurements of PM across the continental U.S. using aerosol water. My field measurements of aerosol water found a minimum of 7% water in particles by volume and a diel cycle in water content controlled, in part, by aerosol hygroscopicity, that ranged from 1-5 µg m-3. Next, semi-observational estimates of aerosol water by an inorganic thermodynamic model discovered that aerosol water mass has decreased in the Southeast by 79% between 2001-2012. Similar reductions of biogenically derived SOA mass in the area are consistent with an aerosol water-mediated mechanism and the correlation in water and SOA mass is statistically robust. Finally, estimates of aerosol water across the continental U.S. yield seasonal differences that were highest in the Southeast and modulated by aerosol chemistry and not RH. Near surface trends in aerosol water are qualitatively similar to space-based aerosol optical thickness measurements and potentially resolve surface and satellite PM measurements. Ultimately, this work contributes to improving our understanding of the connections among aerosol water, biogenic and anthropogenic emissions, and biogenic SOA formed in the presence of anthropogenic perturbations.Ph.D.Includes bibliographical referencesby Thien Khoi V. Nguye